| Author: | Li, Liuqing |
| Title: | Design of efficient ruthenium-based catalysts and study on their catalytic mechanism towards water splitting |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Ruthenium Hydrogen evolution reaction Electrocatalysis Photoelectrochemistry Water -- Electrolysis Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xxvi, 189 pages : color illustrations |
| Language: | English |
| Abstract: | The combustion of conventional energy sources has resulted in significant environmental challenges, including air pollution and climate change. To address these issues, hydrogen energy has emerged as a promising emission-free alternative, offering potential solutions to mitigate climate change and improve environmental conditions. Precious metal catalysts such as iridium oxide (IrO2) and platinum (Pt) have been widely recognized as optimal choices for catalyzing the water-splitting process, either at the anode or the cathode. Unfortunately, the high cost associated with these materials has limited their widespread industrial application. In light of this challenge, the present study aims to investigate the use of ruthenium (Ru), a more cost-effective element with comparable catalytic activity to Ir and Pt, to develop a highly active and economically viable catalyst for potential implementation in industrial-scale water-splitting operations. (i) To replace Pt in catalyzing hydrogen evolution reaction (HER), a series of NiCoRux/Super P (denoted as NiCoRux/SP, x=0.05, 0.1, 0.2 and 0.3) nanocomposites are synthesized to determine the optimal Ru doping ratio in NiCo alloys. It is found that the NiCoRu0.2/SP electrocatalyst achieved an overpotential of 59 mV at 10 mA cm-2 (η10), while excessive Ru incorporation (NiCoRu0.3/SP) diminishes the HER activity. To further study the hidden mechanism, density functional theory (DFT) calculations are carried out to reveal the behavior of ruthenium-doped nickel cobalt alloy when catalyzing the alkaline hydrogen evolution reaction. According to the DFT calculations, a synergistic effect at the Ru-Ni/Co interface is found to accelerate water dissociation and optimize the adsorption-desorption energetics toward the H intermediate. More importantly, excessive Ru introduction will lead to over-strong hydrogen adsorption on the catalyst surface, thus limiting H2 release. X-ray absorption spectroscopy (XAS) and other characterizations further confirm that the interface-induced electron transfer from atomic Ru to its surrounding Ni/Co, and the activity degradation formed by excessive Ru incorporation is attributed to the generation of Ru cluster that sacrifices the interface between Ru atom and Ni/Co. (ii) Ruthenium-based materials have been regarded as promising electrocatalysts to replace the expensive and scarce iridium for the oxygen evolution reaction (OER) in acidic electrolytes, but are limited by their poor stability. Here, we introduce an antimony (Sb) stabilization strategy on ruthenium oxide (RuO2) electrocatalyst to realize simultaneously enhanced efficiency and stability of OER catalyst in acidic media. The optimized Sb0.2Ru0.8O2 electrode can achieve a current density of 10 mA cm-2 at a low overpotential of 220 mV and operate stably for 150 hours. When employed in the proton exchange membrane (PEM) device, the electrode works for 100 hours with little attenuation at 100 mA cm-2. XAS results confirm that introducing Sb can lower the valence state of Ru in RuO2 and suppress the generation of dissoluble products (Ru≥4+ ) in OER condition. Moreover, DFT simulations systematically uncover the origin of the superior activity and robust stability, that is, Sb doping can lower the absorption energy of oxy-hydroxide (OOH*), resulting in the promoted OER activity. Moreover, the doped Sb is evidenced to enlarge the energy barrier of losing surface O and Ru of RuO2, thus fixing the catalyst structure and improving the stability. (iii) Pyrochlore Y2Ru2CrO7 has garnered attention as a promising catalyst for the OER due to its lower Ru mass ratio (20.5 wt.%) compared to RuO2 (75.9 wt.%). To address the inherent instability of pyrochlore, chromium (Cr) is incorporated into the pyrochlore structure to form Y2Ru2-xCrxO7 catalysts with varying compositions (x=0.1, 0.2, 0.4, 0.6 and 1). In 0.5 M H2SO4 electrolyte, the Y2Ru1.8Cr0.2O7 sample exhibited an η10 of 250 mV and maintained stable for 30 hours at a current density of 10 mA cm-2. XAS and X-ray photoelectron spectroscopy (XPS) analysis reveal that the introduction of Cr can reduce the valence state of Ru in Y2Ru1.8Cr0.2O7. What is more, Cr can maintain Ru at a low valence state under OER potential, protecting Ru from being oxidized into soluble RuO42- and thus improving the stability of the catalyst. DFT computing data demonstrate that Cr-dopant can lower the energy barrier of the rate-determining step (O* to *OOH), helping oxygen release. Besides, doping Cr strengthens the Ru-O in the catalysts and enlarges the free energy of losing lattice oxygen and Ru demetallation, which protects the crystal structure of Y2Ru1.8Cr0.2O7 in acidic OER. |
| Rights: | All rights reserved |
| Access: | open access |
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